1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 + TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU02/4%#4)/.G0G0!).34G0G0).4%2 the higher the voltage of the power line, the shorter the length of the cable section, and the higherthe resistivity of the soil, the greater is the danger. This dangerous situati
2、on also arises whenever an earth fault occurson a pylon near the cable.If circumstances require the overhead line to be routed in a cable, special precautions will have to be taken atthe crossing, for example: the use of an insulating covering around the metal sheath of the cable; the use of a cable
3、 with an all-plastic sheath.2 Line left overheadThe method whereby the power line is separated from the telecommunication line by a guard-wire or a cradlecannot generally be recommended.In any case, regardless of the circumstances, a minimum vertical distance has to be kept betweentelecommunication
4、conductors, in conformity with national regulations.There are, moreover a number of arrangements that could be introduced to reduce the danger:2.1 Use of a common support at the crossing-point, provided the insulators used for the telecommunication linehave, if necessary, a high breakdown voltage.2.
5、2 Insulation of the conductors, preferably the telecommunication conductors, provided that such insulation isproperly adapted to the conditions existing.2.3 Reinforcement of the construction of the power line where the crossing takes place, so as to minimize the riskof a break.3 Circumstances in whi
6、ch the various arrangements in 2.1, 2.2 and 2.3 above are applicableThe application of these methods depends primarily on the voltage of the power line. The voltage ranges to betaken into account are not related to the International Electrotechnical Commission (IEC) standardization, because ofthe sp
7、ecial features of the problem raised.3.1 Systems using voltages of 600 V or lessArrangements to be as in 2.1 and/or 2.2.2 Volume IX - Rec. K.63.2 Systems using voltages of 60 kV or more(In particular the “high reliability” system referred to in 1.)Arrangements to be as in 2.3, if necessary.3.3 Inter
8、mediate voltage systemsFor the 600-V to 60-kV range, because of the variety of voltages, the mechanical characteristics of lines and theoperating methods encountered, it is impossible to issue precise recommendations.However, one or more of the arrangements described above might be applicable, although certain special casescall for thorough examination in close collaboration with the services concerned.Reference1 CCITT manual Directives concerning the protection of telecommunication lines against harmful effects fromelectric power and electrified railway lines, Vol. VI, ITU, Geneva, 1988.
